Some of my event transactions are showing "Pending (incomplete transaction)" when they have been successfully processed through Paypal. Not all of them just two since my event went live. I'm on Joomla! 3.6.5 and CiviCRM 4.7.16
We had trouble with this, it could be related to your IPN listener in paypal, (Log on to paypal, go to profile, my selling prefs, Instant payment notifications to check out what's going on - if it looks fine, send paypal a message)
Or you should certainly double check that you've got the correct paypal account details saved within the Payment Processor in Civi.
We had loads of trouble figuring out why our payments were not completing, and in the end it boiled down to which paypal email address was being referenced in the payment processor - the default email on paypal had been changed.